1. Technology Field
The present invention generally relates to receivers used to detect optical signals in an optical communications network. In particular, the present invention relates to a discrete bootstrapping configuration for an optical receiver that reduces the incidence of feedback of a signal received by the optical receiver.
2. The Related Technology
Fiber-optics and optoelectronics are important aspects of modern optical networks because they allow for efficient, accurate and rapid transmission of optical data between various components in the network system. An optical transceiver module (“transceiver”) is an example of a modular component that is used in optical networks. Such modular components are desirable in optical networks and other fiber optic systems to reduce the cost of manufacturing the system, which cost increases the more customized the system becomes.
Transceivers usually include an input receiver optical subassembly (“ROSA”) and an output transmitter optical subassembly (“TOSA”). The ROSA includes a photodiode or other optical detector for detecting optical signals and sensing circuitry for converting the optical signals to electrical signals compatible with other network components. The TOSA includes a laser or other suitable light source for transmitting optical signals and may include control circuitry for modulating the laser according to an input digital data signal and a photodetector to monitor laser power.
The TOSA has an optical lens for focusing the optical signals from the laser of the TOSA into an optical fiber. Similarly, the ROSA often includes a lens to focus incoming optical signals on the photodiode. Additionally, one end of the transceiver includes pluggable receptacles, pig-tailed connections, or other suitable means for optically connecting the TOSA and the ROSA with other components within a fiber optic network, while an opposite end of the transceiver includes a connector for connecting with electrical components of a host system or device with which the transceiver communicates.
The photodiode in the ROSA and the laser in the TOSA are examples of optoelectronic semiconductor components. Generally, these optoelectronic semiconductor components are sensitive devices that require mechanical and environmental protection. As such, these optoelectronic components are usually manufactured in packages to provide such protection and to facilitate their incorporation into higher level devices, such as TOSAs and ROSAs.
One such packaging assembly is known as a transistor-outline package, referred to herein as a “TO package.” TO packages are widely used in the field of optoelectronics, and may be employed in a variety of applications. As such, TO packages are often standardized to facilitate their incorporation into components such as transceivers. The TO packages protect the sensitive electrical devices contained therein and electrically connect such devices to external components such as printed circuit boards (“PCB”).
With respect to their construction, the TO packages often include a cylindrical metallic base, also known as a header, with a number of conductive leads extending completely through, and generally perpendicular to, the base. The size of the base is often sized to fit within a specific TO standard size and lead configuration, examples of which include a TO-5 or TO-46. The leads are usually hermetically sealed in the base to provide mechanical and environmental protection for the components contained in the TO package, and to electrically isolate the conductive leads from the metallic material of the base. Typically, one of the conductive leads is a ground lead that may be electrically connected directly to the base.
Various types of electrical devices and optical components, such as the photodiode or laser device, are mounted on an interior portion of the base and connected to the leads to enable their operation. Generally a cap, also known as a can, is used to enclose the interior portion of the base where such electrical devices are mounted so as to form a hermetic chamber that helps prevent contamination or damage to the devices. The specific design of the TO package depends on the optoelectronic component being mounted on the base and the modular component with which the TO package will be used. By way of example, in applications where the optoelectronic component mounted on the base is an optical component, i.e., a laser or photodiode, the cap is at least partially transparent so as to allow an optical signal generated or received by the optical component to be transmitted to or from the TO package. These optical TO can packages are also known as window cans.
As stated above, optical receivers are specifically built for the purpose of receiving and interpreting light signals. An optical receiver typically includes some sort of detector that can generate an electrical current or voltage in response to changes in the power of the incident optical signal. After the fiber optic receiver converts the optical signal received over the optical fiber into an electrical signal, the optical receiver amplifies the electrical signal, and converts the electrical signal into an electrical digital data stream.
One of the common devices used as a detector in an optical receiver is a photodiode. A photodiode operates by generating a current in response to incident light. The optical power of the incident light determines the current that flows in the photodiode. In effect, the optical signal generates current in the photodiode that corresponds to the digital data carried by the optical fiber.
Despite their utility, packages such as TO packages that house photodiodes or other optical detectors can suffer from performance-related challenges. One of these challenges is signal feedback. In the case of optical receiver packages, feedback is a result of amplification of the electrical signal converted from an optical signal received by the photodiode, as explained above. This amplification is performed by a signal amplifier, such as a transimpedance amplifier, and the amplification of the output signal produced by the amplifier can be significant when compared to the original strength of the converted photodiode signal, which can result in a certain amount of feedback. Moreover, the signals converted by the photodiode are often high frequency signals of 10 GHz or more, which can further exacerbate feedback.
Thus, significant signal amplification, together with the high frequency of the amplified signal, combine to create a signal that is apt to produce feedback in the system in which the photodiode and amplifier are found. This feedback is manifested as a portion of the signal from the amplifier ground that migrates back to the amplifier input via various structures, including the header surface, power supply and ground connections, bond wires, etc. Such feedback is unintended and can represent a significant limitation in terms of performance of the package, e.g., frequency response. Should the feedback exceed minimal levels, oscillation can occur, which undesirably destroys any functionality of the package and requires scrapping of the part.
In light of the above, a need exists for controlling feedback in an optical receiver system, such as an optoelectronic package housing a photodiode, in order to optimize operation of the device. Any solution should be implemented in a manner that does not substantially increase the sophistication or complexity of the device and that does not compromise signal integrity.